Every weekday, the London Underground sees up to five million passengers on and off its network. Its 11 lines serve 272 stations and, at peak times, more than 500 trains run under the streets of London.
Despite its constant use since the 19and Century, the London Underground has been relatively little studied. While other modes of transportation have been researched and updated accordingly – think back to the days when we didn’t have seatbelts in cars or airbags – the sprawling tube system has remained in large party the same.
The pandemic has given Transport for London (TfL), which runs the Tube, the opportunity to make some improvements. The ventilation systems have been evaluated and according to the 2021 Mayor’s Transportation Strategy Update “London Underground’s ventilation infrastructure is generally designed beyond legal minimum requirements with an adequate supply of fresh air.”
But how cool is the air you breathe in the tube?
“The air, even before it arrives in the subway, is not perfectly pure,” explained Dr David Green, who leads the aerosol science team at Imperial College London and is a member of the UK Committee on the Medical Effects of Air Pollution (COMEAP). Green is also part of a group commissioned by TfL to regularly assess COVID-19 risk on the Tube.
“Urban background air already has a low level of particulate matter, but on top of that you have all these additional emissions [coming from the tube].”
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These include particles from the carriage moving along the rails, brake pads rubbing on the wheels, and the electrical connection between the collector plate and the live rail.
“So you have iron rubbing on steel, steel on steel, iron and copper, and barium coming from brakemen,” Green says. “There is lubricant on the wheels and it contains things like carbon and molybdenum. These metal compounds are found in the atmosphere [on the Underground].”
There are also particles that come from the tube passengers, humans and others. Hair and skin cells, plastic fibers from clothing, and animal particles from creatures that live underground all contribute to air quality.
Some particles are large enough to be caught by the hairs of our nose and throat, preventing them from entering our lungs and causing damage there. These are usually referred to as PMten; particles less than 10 micrometers in diameter, or about 0.01 mm.
The smallest particles less than 2.5 micrometers in diameter are called PM2.5, and these are the ones that can penetrate deep into the lungs and can also enter the bloodstream to be carried around the body, affecting the brain, heart and other organs. These are about 3 percent of the diameter of a human hair.
“We don’t have enough evidence to say categorically whether or not pollution on the tube is harmful to your health, but we do know that exposure to PM2.5 is harmful. [This is shown by] studies from around the world looking at deaths and hospital admissions, and studies of smaller groups of people, including those with existing health conditions.
“However, dust in subway systems is quite different from PM2.5 in the outdoor air and we don’t know if we can extrapolate these results to the metro environment. We are therefore currently studying vulnerable groups of people and TfL staff to understand if exposure to this type of PM2.5 is harmful. »
In addition to these small particles, there are also gaseous pollutants in the air, including ozone, O3and nitrogen dioxide, NO2. All are monitored by the UK Department for Environment, Food and Rural Affairs, whose 2020 publication revealed there had been a long-term reduction of all pollutants measure.
However, UK values for PM2.5 are even higher than the guidelines established by the World Health Organizationwhich have recently changed their goal for an average annual exposure not exceeding 5 micrograms per cubic meter (μg/m3). Previously it was 10 μg/m3. The European Union limit is 25 μg/m3which is also the limit set by UK law.
“The new WHO guideline is very difficult and currently I don’t think any place in the UK will respond to this,” says Green. A 2020 UK government study found that four of the top five urban environments with largest annual PM2.5 values were located in London, South East or East England Regions. According to the latest COMEAP study, the concentrations of PM2.5 in the London Underground were several times greater than in other London transport environments, and greater than in other underground systems worldwide.
However, when it comes to air quality in the subway, Green says it’s important to consider context across the range of transportation modes. A report in 2021 compared the PM2.5 averages through the metro, bus, car, the three types of trains, cycling and walking.
Interestingly, the lowest exposures were seen on electric and hybrid trains, even when compared to cycling and walking – although this was not the case when these trains were in stations next to motor trains diesel.
“[Travelling on the tube] for one hour every weekday for 48 weeks a year (assuming 4 weeks holiday) on the Victoria Line would increase your annual PM exposure2.5 6.8 μg/m3“says Green. This increase is in addition to the usual exposure of individuals to pollutants in the air, which varies by location, as some of the small particles measured in PM2.5 occur naturally, like dust and sea salt.
“This compares to 0.3 μg/m3 at a background site in London, 2.6 μg/m3 on an average London Underground line or 1.2 μg/m3 in a car. But remember that the car also pollutes everyone.
For Green, this is the key. “It’s much better for people to take the London Underground than to get in their car to get around London. Indeed, if you are sitting in your car, you are exposed to very high concentrations of automotive pollutants.
“You’re sitting directly behind the car’s exhaust [in front], you are therefore more exposed than cyclists traveling along the road or pedestrians passing in front of it. And the other thing is that you also pollute the world for everyone. So while the car is no worse than the tube in the case of PM2.5it is much worse for other pollutants such as nitrogen oxides.
For those who need to travel by metro, it is then a question of choosing the best route. Green’s research found that deeper lines are generally worse, like the Northern line, compared to higher level lines like the Circle, District, Hammersmith & City and Metropolitan lines. The age of the lines and the type of trains used all play a role. So, Green suggests opting for newer lines and stations that have platform gates installed to reduce exposure. “But in general, there’s not a lot of choice,” he admits.
Green is also concerned about the health of London Underground staff, who are spending far more time on the tube or in a station. Unfortunately, there is not yet enough data to say what the long-term health effects will be.
“We are working closely with Transport for London (TFL) to compare the sickness absence of people working on the London Underground with other TFL workers. We also want to look at the pension data, to see if the people who work on the tube may die a little earlier than the others. Corn [these studies] are in the early stages at the moment.
Subway passengers can be reassured that the risk of catching COVID-19 is however minimal throughout the subway.
“Tube trains and stations are cleaned with hospital grade cleaning substances which kill viruses and bacteria on contact and provide ongoing protection,” a TfL spokesperson said. BBC Science Focus magazine.
“Independent testing by Imperial College London has been carried out monthly since September 2020, taking swabs from touchpoints at stations, on buses and air samples from ticketing halls and up to the last round of tests verified in December 2021 found no trace of coronavirus on the public transport network. »
About our expert
Dr David Green is a senior researcher at Imperial College London, where he leads the aerosol science team. His research focuses on particles in the air and their effects on the health of populations.
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